1. Field of the Invention
The present invention relates to an image display apparatus. More particularly, the present invention relates to a projection image display apparatus.
2. Description of the Related Art
Conventionally, there is known a projection image display apparatus which includes a light source, an optical system for separating illuminating light beams from the light source according to polarization and composing illuminating light beams, an image display element for converting the emitted light beams into image light beams, and a projecting optical system for forming an image with the converted image light beams.
In the following, a typical construction of a projection image display apparatus is described. White light emitted from the light source is arranged in a predetermined polarization state by a polarization conversion element, and is separated into green light and blue and red light by a dichroic mirror. Polarization directions of the blue and red light in predetermined wavelength bands are further rotated by 90 degrees by a color selection retardation plate, and light of a predetermined color illuminates an image display element corresponding to the predetermined color via a polarizing beam splitter (PBS) and a retardation plate. The image display element converts the illuminating light into image light according to an image signal and reflects the image light. The image light is composed by the polarizing beam splitter, a composition prism, and the like, and is projected onto a screen by a projecting optical system.
By the way, generally, in a projection image display apparatus, even when the image display element is in a black display state, leak light on a side of the projecting optical system lowers contrast of an image. Among elements included in an image display apparatus, a polarizing beam splitter and a retardation plate have polarization characteristics which depend on the incident angle, and thus, the characteristics of polarized light which passes through or is reflected by the element vary depending on the incident angle. In other words, phase shift is caused according to the incident angle. Therefore, the polarizing beam splitter may not uniformly polarize and separate all light, and part of the light becomes leak light.
Pixels of the image display element are arranged at regular intervals on the order of several micrometers. When incident light is reflected by the image display element, diffracted light according to the shape of an aperture is generated. The diffracted light is reflected at an angle which is different from that in the case of normal reflected light (0-th order reflected light) according to the order of diffraction thereof, and thus, optical characteristics of the reflected diffracted light received by a polarization element differs from those of normal reflected light received by the polarization element. As a result, a majority of the diffracted light is out of a desired polarization state and polarization and separation are not carried out enough, which is a factor to cause leak light.
Even when the image display element is in the black display state, such light is projected onto the screen via the polarizing beam splitter and a projection lens, which is a cause of the lowered contrast of the displayed image.
With respect to the above-mentioned problem, Japanese Patent Publication No. H07-038050 describes a method in which leak light is compensated for by disposing a ¼ wavelength plate between a polarizing beam splitter and a reflection type image display element. The incident light passes through the ¼ wavelength plate in a forward path and a backward path during reciprocating between the polarizing beam splitter and the reflection type image display element, so that leak light due to the dependence of the polarizing beam splitter on the incident angle is compensated for.
US 2006/0285042 A describes compensation for nonideal polarization effects of a polarizing beam splitter or a liquid crystal display element by disposing in an optical path a liquid crystal display element and a compensator having the optic axis which is tilted with respect to the liquid crystal display element.
In the method of compensating for the incident angle characteristics of the polarizing beam splitter by passing light through the ¼ wavelength plate in the forward path and the backward path, although light which is normally reflected (0-th order reflection) by the reflection type image display element may be compensated for, diffracted light may not be compensated for. Because diffracted light is reflected at an angle which is different from the incident angle, the angle of the incident light is different from the angle of the reflected light in the forward path and the backward path through the ¼ wavelength plate. Such light may not obtain the compensation effects by passing through the ¼ wavelength plate in the forward path and the backward path.